The U.S. stock market had its best start to a year since 1999, but by Friday—the halfway mark of 2013—investors had ditched their party hats and braced for the Federal Reserve to cut back on policies that helped send stocks soaring this year.

The Dow Jones Industrial Average ended the first six months of the year up 14%, but all the gains came in the first five months. The Dow fell 1.4% in June, including a 114.89 point, or 0.76%,...